This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-320004, filed Nov. 10, 1999, the entire contents of which are incorporated herein by reference.
This invention relates generally to a head positioning control system for a disk drive, and more particularly to a head positioning control system using a sampled data control system.
A conventional disk drive, such as a hard disk drive (HDD), has been provided with a head positioning control system (also referred to as a servo system) for positioning the head in the target position (or on the track to be accessed) on a disk serving as a data recording medium.
The head positioning control system is composed of a digital control system called a sampled data control system. The sampled data control system is a system that causes a controller (a discrete system) operating in discrete time to control a plant (an object of control) operating in continuous time. Specifically, the output (the state of the position or speed) of the plant is sampled (observed) at regular intervals of time (in a sampling period). In synchronization with the sampling period, the controller (actually, a CPU) calculates the control value (or the control input value for controlling the plant). In other words, the system switches control values according to the state of the plant observed by the controller in the sampling period, thereby driving the plant continuously.
In a HDD, the plant is an actuator including a head and a voice coil motor (VCM) and, in a narrow sense, corresponds to a VCM. The CPU calculates the position and travel speed of the head on the basis of the servo data read by the head from the disk. In HDD, the CPU performs seek control (also called speed control) whereby the head is moved to the target position (desired track) on the disk and track following control whereby the head is settled (kept) in the range of the desired track which the head has approached or reached.
The sampled data control systems are roughly divided into single-rate control systems and multi-rate control system. The single-rate control system is such that the sampling period coincides with the period in which the controller outputs the control value. In other words, the single-rate control system calculates one control value in a single sampling period and outputs the value to the plant. On the other hand, the multi-rate control system calculates control values in a single sampling period and switches between the control values.
FIG. 8 is a block diagram of a single-rate control system applied to seek control in head positioning control. The control system is provided with an observer 2 that has the function of observing the head position (HP) in a specific period (sampling period) on the basis of the output (position or speed) of the plant 1 and estimating the travel speed of the head at the time when the position was observed (or at the sampling time). In short, the observer 2 corresponds to an estimator that estimates the position or speed on the basis of the result of observing the state of the plant 1.
The difference between the target data (target position and target speed) from a reference input unit 4 and the result of observation (head position and travel speed) from the observer 2 is inputted to a controller 3. The controller 3 calculates the control value (u) by adding a correction value for correcting the difference and a specific input value (a type of compensation value) 5 and drives the plant 1 on the basis of the control value. The reference input unit 4 corresponds to an input section to which a state command is inputted in a feedback control system.
FIG. 9 is a block diagram of a multi-rate control system applied to the seek control. The control system is provided with an estimator model 6 that estimates the state (position and speed) of the plant 1 between sampling times, the times when the observer 2 makes observations. The estimator model 6 estimates the state (head position and travel speed) of the plant 1 between sampling times from the result (head position and travel speed) of the observations made by the observer 2 at sampling times. On the basis of the result of the estimation, the controller 3 calculates control values (u) in a single sampling period and switches between the control values at intervals of one sampling period, thereby performing driving control of the plant 1. Since such a multi-rate control system can perform finer control than the single-rate control system, it can perform seek control that responds faster to disturbance particularly affecting control in operation.
FIG. 10 shows a seek trajectory of the head in a single track range (with the track center, the target position, being 0) controlled by the head positioning control system. In FIG. 10, the characteristic when disturbance (for example, an impact) was applied temporarily to the system is shown. Specifically, in a seek trajectory 100 by the single-rate control system, the effect of disturbance cannot be absorbed and the amount of overshoot (the amount of shift in the head position) with respect to the target position (0) is large. In contrast, in a seek trajectory 101 by the multi-rate control system, response to disturbance is fast and therefore the amount of overshoot is relatively small.
After control has changed from the seek control performed up to the desired track to the track following control performed in the range of the desired track, the reference input unit 4 inputs the target position to the controller 3. The specific input value (5) is 0.
To the head positioning control system in a conventional HDD, a multi-rate control system particularly excellent in the performance of seek control has been applied. Since the multi-rate control system calculates control values in a single sampling period as described above, the amount of calculations done by the CPU are much larger than in the single-rate control system.
The estimation of the state (the estimation of position and speed) between sampling times depends on the accuracy of the estimator model 6. Thus, when the difference between the dynamic characteristic of the plant 1 and the result of estimation by the estimator model 6 is great, there is a strong possibility that the controller 3 will calculate an oscillatory control value. In addition, because in the multi-rate control system, the control values are switched in a shorter time than the sampling period (or has a shorter control period), when a large control value is encountered, the control input is more liable to be saturated than in the single-rate control system. The saturation of the control input means the upper limit of the control value set on the basis of the dynamic characteristic of the plant 1. Specifically, the control value calculated by the controller 2 has reached the upper limit. Thus, when the control input is saturated, the system is impossible to control.
In short, the head positioning control system using only the multi-rate control system has the following problems: the amount of calculations done by the CPU increases, the estimation of states depends on the accuracy of the estimator model, and the control input tends to be saturated. Thus, taking into account the environment in which the HDD is used, the head positioning control system to which the multi-rate control system has been applied has an insufficient performance particularly in terms of stability.
The object of the present invention is to provide a high-performance head positioning control system which has overcome the problems encountered in using only a multi-rate control system.
The head positioning control system of the present invention is so constructed that a multi-rate control system and a single-rate control system are combined suitably and that it switches between the multi-rate control system and the single-rate control system according to a specific condition in the control process.
Specifically, the system of the present invention, which is applicable to a disk drive, comprises actuator means for moving the head to a target position on a disk and positioning the head in the target position; observer means for observing the state of the position or travel speed of the head on the disk in a specific sampling period; single-rate control means for calculating, in synchronization with each sampling time in the sampling period, a control value for performing positioning control of the head in the target position on the disk on the basis of the result of observation by the observer means; multi-rate control means for estimating the state of the head between the sampling times in the sampling period on the basis of the result of observation by the observer means and calculating a plurality of control values in a single sampling period using the result of the estimation; and switching control means for switching between the single-rate control means and the multi-rate control means under the condition for the process of positioning the head in the target position and inputting the control value calculated by either the single-rate control means or the multi-rate control means to the actuator means.
In other words, the single-rate control system corresponds to a minimum multi-rate control system. In the single-rate control system, the plant observation period coincides with the period of the control input (the calculation period of the control value) or one control value is calculated through one head position observation.
The actuator means corresponds to an actuator mechanism including a head and a VCM. The observer means estimates and observes the position or travel speed of the head on the disk in the specific sampling period. In the disk drive, a CPU constituting the controller executes the single-rate control means, multi-rate control means, and switching control means.
With such a configuration, the present system uses the multi-rate control system in seek control whereby, for example, the head is moved to the target track, and changes the system to the single-rate control system when changing from the seek control to track following control. Specifically, the multi-rate control system excellent in response to disturbance causes the head to reach the target track at high speed. After the head has reached the target track, the system is changed to the single-rate control system with a small amount of calculations done by the CPU. Then, the single-rate control system performs the track following control. This not only realizes high-speed seek control but also decreases the load on the CPU in track following control, which improves the efficiency of the CPU in processing data except for servo control.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.